Contractile, electrophysiological, and biochemical characteristics are studied in dissociated cardiac cells from adult rats, rabbits and guinea pigs. We have developed a system for simultaneous measurement of changes in cytosolic free (Ca2+) cell length and membrane current/voltage, with high time resolution. The system uses the fluorescent probe Indo-1 to monitor cytosolic free (Ca2+) transient. Cell length is measured from the bright-fieid image of the cell by an optical edge tracking method using a video edge detector and the membrane potential can be monitored simultaneously with patch electrodes. Salient recent results include the demonstration that (1) Ca currents rather than depolarization per se or other depolarization mediated events are required to trigger Ca2+ release from the sarcoplasmic reticulum to cause the rapid increase in cytosolic (Ca2+) that initiates the twitch contraction; (2) the slow terminal relengthening phase of a twitch contraction coincides in time with a slow decay of cytosolic (Ca2+); (3) prolonging the duration of depolarization reverses the negative staircase of cytosolic (Ca2+) transients and contraction observed upon stimulation of rat myocytes from rest, presumably by augmenting the cell Ca2+ load.